This invention relates generally to mattress foundation structures and more particularly to a box spring assembly of a type which utilizes non-coil springs. Box spring assemblies of this general type have been known since 1964, the first such spring assembly being disclosed in U.S. Pat. No. 3,286,281. Subsequently issued patents disclosing the same general type of box spring assembly are: U.S. Pat. Nos. 3,487,480; 3,506,987; 3,574,240; 3,574,241; 3,665,529; 3,680,157; 3,755,833; 3,824,639; 3,852,838; 4,060,862; 4,120,058; 4,131,961; 4,195,376; 4,218,790; 4,238,861; 4,251,892; 4,253,208; 4,339,834; and 4,470,584. Box spring assemblies of the general type shown in the above list of patents, all of which are owned by the assignee of this application, are advantageous with respect to the conventional box spring assemblies using coil springs because they provide a desired stiffer foundation for the mattress and contain a reduced amount of wire. These box spring assemblies are also advantageous from the standpoints of prolonged service life, ease of assembly, and cost of manufacture.
Additional box spring assemblies of this general type are shown in U.S. Pat. Nos. 3,546,723; 3,596,299; 3,722,013; 3,825,960; 3,833,948; 3,835,485; 3,869,740; 3,990,121; and 4,000,531.
Some current box spring assemblies typically have a wire grid assembly which is supported by an array of spring units attached to a supporting frame. One approach toward reducing costs of the box spring assembly is to minimize the gauge of the wires making up the grid assembly. Grid assembly wire diameter, however, is dependent upon the characteristics of the spring units on which the grid is supported. Some spring unit types do not distribute their loads on the grid assembly but instead exert undesirable localized or "point" loads which tend to permanently deform portions of the grid assembly when it is subjected to bedding loads. For assemblies employing such spring unit types, heavy gauge wire is necessary in the grid assembly to resist deformation.
Accordingly, it is desirable to provide an improved spring module or unit which reduces localized loading on the grid assembly. As a means of simplifying fabrication, it is further desirable to provide a spring module which can be conveniently attached to both the box spring frame structure and the grid assembly. Since box spring assemblies may be subjected to overload conditions, the spring units should further be capable of withstanding such loading without sustaining permanent damage or taking a set. Therefore, the springs of a box spring assembly must include means for preventing deflection beyond elastic limit. Since large numbers of spring modules must be handled and packaged, it is further preferable for them to be stackable in a compact manner without interlacing or twisting.